1. Field of the Invention
The present invention relates to a respiratory gas concentration measuring apparatus for measuring the concentrations of a gas, such as CO.sub.2, in respiratory gas based on the absorption of gases to be measured with respect to light of a particular wavelength which passes therethrough.
2. Description of the Prior Art
U.S. Pat. No. 4,522,204, the disclosure of which is incorporated herein by reference, describes a two-wavelength type respiratory gas concentration measuring apparatus in which infrared light rays are generated to pass transversely through a tube through which respiratory gas passes longitudinally. Two different filters are alternately positioned in the light path. The filters are situated so that the tube containing the respiratory gas is located between the light source and the filters. One of the filters allows the transmission of a light component whose wavelength will be absorbed by a particular gas, while the other filter allows the transmission of a light component whose wavelength will not be absorbed by the same gas. The infrared rays transmitted through these filters are detected by a photodetector employing a PbSe infrared-ray sensor.
The photodetector provides an output signal corresponding to the light transmitted through each filter to a first and a second detector. Based on the signal from the photodetector, the first detector effectively detects the light output of the filter which allows the transmission of the absorption wavelength light component, and the second detector effectively detects the light output of the filter which allows the transmission of the non-absorption wavelength light component. The concentration of a gas, such as CO.sub.2, contained in a respiratory gas is measured on the basis of the ratio of the outputs of the two detectors.
Based upon the ratio between the output Vs of the first detector, which detects the absorbed wavelength light component, and the output Vc of the second detector, which detects the non-absorbed wavelength light component, (i.e., Vs/Vc), an apparatus as described above makes it possible to obtain a gas concentration measurement in which errors due to variations in the intensity of the light source and in the sensitivity of the measuring devices are compensated for. However, if temperature-dependent coefficients of the detected outputs dependent on light rays of the different wavelengths (absorbed wavelength and unabsorbed wavelength) are different from each other due to variations in temperature or the characteristics of the infrared-ray detecting element, temperature dependent drifts will still occur.
In view of the above, the applicants of the present application have proposed in U.S. Pat. No. 4,522,204 a two-wavelength type respiratory gas concentration measuring apparatus. This instrument was based on the finding that the temperature-dependent drifts in detector outputs due to the components of the apparatus from the light source to the first and second detectors have an exponential relationship to temperature-dependent coefficients inherent in the wavelengths at least in the temperature range of 10.degree. C. to 40.degree. C. The proposed apparatus is equipped with a power computing means. Assuming the temperature-dependent coefficient of the first detector output is .theta.1 and the temperature-dependent coefficient of the second detector output is .theta.2, the power computing means calculates Vs/Vc.sup.m (where m=.theta.1/.theta.2). This computation eliminates any influence due to the difference in the temperature-dependent coefficients.
Applicants of the present application have also proposed in co-pending U.S. application Ser. No. 07/462,228, filed Jan. 9, 1990, the disclosure of which is also incorporated herein by reference, a photodetector having a PbSe infrared-ray sensor in which the effect of the temperature fluctuation on the dark resistance of the PbSe infrared-ray sensor is taken into account. Thus, instead of providing a conventional constant-current type photodetector, a constant-voltage type photodetector is used in which the current that flows when a constant voltage Vx is applied to the PbSe infrared-ray sensor 41 is measured, as shown in FIG. 3, thereby making it possible to obtain an excellent temperature compensation characteristic.
Thus, assuming the voltage of the constant-voltage source is Vx, the variation in resistance when the PbSe infrared-ray sensor 41 receives infrared rays is .DELTA.R, and its dark resistance is Rd, the A.C. infrared-ray-detection signal Iout (AC) will be as follows: ##EQU1##
Thus, it is ascertained that the temperature-dependent coefficient of .DELTA.R corresponds, at least at normal temperatures, approximately to the square of the temperature-dependent coefficient of Rd. Since the temperature-dependent coefficient of .DELTA.R/Rd.sup.2 is approximately 0, this makes it possible to output an infrared-ray-detection signal which is determined by a Vx that is stable with respect to temperature. Assuming the resistance of a feedback resistor 42 is Rf, the A.C. voltage infrared-red-detection signal Vout (AC) will be as follows: ##EQU2##
Thus, an infrared-ray-detection signal which is more stable with respect to fluctuations in temperature can be obtained.
However, when a constant-voltage type infrared detector, such as disclosed in U.S. application Ser. No. 07/462,228 mentioned previously, is used in a two-wavelength type respiratory gas concentration measuring apparatus, such as described in U.S. Pat. No. 4,522,204, it is not sufficient to use just the compensation technique provided by the power computation circuit disclosed in the aforementioned U.S. patent, as that technique was more specifically designed for use with a respiratory gas concentration measuring apparatus employing a constant-current type photodetector.